Nanoparticle films are useful as building blocks for complex structures and devices; thus, it is desirable to create improved methods of forming high quality, robust nanoparticle films. Although various methods are known in the art for forming nanoparticle films, such methods are unable to provide robust patterned nanoparticle films of controllable thickness free of gaps and having a high degree of surface smoothness and uniformity.
Nanoparticle films have been deposited using electrophoresis, where a pair of electrodes having a voltage applied therebetween are immersed in a polar solvent having nanoparticles suspended therein. A nanoparticle film is formed on a surface of one of the electrodes. The nanoparticle films that are formed by electrophoresis are of a poor quality in that the films tend not to be continuous and are limited to a monolayer of nanoparticles in thickness. Moreover, such films are not robust in that when the polarity of the voltage applied to the electrodes is reversed, the deposited nanoparticles go back into the solution. Similarly, when a nanoparticle film deposited by electrophoresis is immersed in the solvent, the film dissolves in the solvent.
Nanoparticle films have also been formed by non-electrodeposition techniques. One such technique is dry casting where a drop of solvent having nanoparticles suspended therein is deposited on a flat substrate surface. When the solvent is allowed to evaporate, a monolayer thick nanoparticle film is formed on the substrate surface by self-assembly.
However, during solvent evaporation in dry casting, the nanoparticles tend to spread, making the uniformity of the resulting film difficult to control. This spreading takes place rapidly when fast-drying solvents, such as hexane, are used. The amount of spreading is lessened when slow-drying solvents (such as heptamethylnonane) are used; however the drying often requires too much time, and residues of the solvent may remain in the film.
Nanoparticle films formed by dry casting tend to have rough surfaces that are not flat. Moreover, the films are not robust in that they dissolve back into solution when immersed in the solvent. In addition, nanoparticle films formed by dry casting are unpatterned. Thus, a nanoparticle film formed by dry casting is generally not useful for certain applications such as use as a layer of an electronic device. Moreover, the thickness of a nanoparticle film formed by dry casting cannot be accurately controlled.
Another known non-electrodeposition technique for forming nanoparticle films is spin coating, where an amount of a suspension comprising a solvent and nanoparticles is placed onto a flat substrate surface, and the substrate is spun very rapidly to cause a flat film of the nanoparticles to be formed on the substrate surface. In some spin coating processes, the nanoparticles are spin coated with polymer precursors to form a composite film of nanoparticles and the polymer on the substrate surface. Like dry casting, the process of spin coating relies on the self-assembly of the nanoparticles to form the layer on the surface of the substrate.
Nanoparticle films formed by spin coating also tend to have rough, non-flat surfaces and are not robust in that the film dissolves back into solution when immersed in the solvent. Likewise, the thickness of the film formed on the substrate surface by spin coating cannot be accurately controlled. Moreover, nanoparticle films formed by spin coating are unpatterned, and thus would not be useful as a layer in an electronic device where a high quality patterned nanoparticle film is needed.
Another process for electrochemical deposition of a nanoparticle film onto a surface of an electrode is disclosed in Zhang et al., Adv. Mater. 11 (17), 1437 (1999). In this process, the reactants for forming the CdSe nanoparticles are mixed in a solution, and a pair of cadmium/gold electrodes are immersed in the solution. When a direct current (DC) voltage is applied between the electrodes, nanoparticles that are formed in the solution are deposited on the surface of one of the electrodes, specifically, the cathode. The deposition of the nanoparticles on the cathode forms a film in that the coverage of the nanoparticles over the cathode surface is substantially complete.
However, the nanoparticle films that are formed using the Zhang et al. process tend to have gaps. Also, in such films, the CdSe nanoparticles have mixed crystal structures in that all three of the CdSe nanoparticle crystalline structures are present in the film. Thus, the film has poor uniformity. Furthermore, the films deposited using this process have rough surfaces and are not robust. Additionally, the Zhang et al. process is not able to form nanoparticle films having more than one layer of the nanoparticles.
Therefore, a need exists in the art for a method of forming unpatterned and patterned nanoparticle films having multiple layers of nanoparticles, where the thickness of the film may be accurately controlled and where the films are uniform, have smooth surfaces, and are robust in that the films do not dissolve when immersed in the solvent.